The present invention relates to the automatic wrapping of cylindrical products, such as rolls of toilet or kitchen paper, in sheets of plastic film suitably folded and sealed in direct contact with the rolls. More specifically, the invention relates to an apparatus for automatically controlling the work flow of a wrapping machine of the type just mentioned.
Automatic wrapping machines of this kind are normally equipped with a set of effectors, that is to say, different parts used to perform the different steps of the operating cycle. These effectors include: an elevator having a table that is reciprocatingly movable in a vertical direction between a lowered position, in which it receives the products to be wrapped, and a raised position, in which the products are positioned inside a wrapping station; a pair of planar horizontal folders located under the wrapping station on each side of a vertical, central machine axis, the pair of planar horizontal folders being reciprocatingly movable in reciprocally opposite horizontal directions; and two pairs of planar vertical, parallel folders, located on each side of the wrapping station and being reciprocatingly movable in reciprocally opposite horizontal directions; the machine being also equipped with a plurality of pushers positioned in succession at equal intervals along a horizontal direction passing through the wrapping station, the pushers being designed to accommodate the products in pairs and to feed them in the horizontal direction, and with rotational motors for driving the effectors.
During the process cycle, the duration of every physical event performed by the effectors corresponds to a certain angle of rotation traveled by the related drive motor or, otherwise, to an angle of rotation traveled by a controlling motor with which the effector drive motors are synchronized and to which they are hierarchically subordinated.
It is therefore possible to set the steps, advances and delays of the effectors in accordance with the angles traveled by the controlling motor and to create a work flow in which all the operating steps are correlated in the required sequence.
In machines of the aforementioned kind, the work flow is based on a product-specific work flow diagram where the angular values determining the transitions from one step to another, once defined and optimized for a specific product, are kept fixed and are invariable over time.
As a general rule, the adaptation of an automatic machine to operate on the basis of a different work flow diagram is possible but requires lengthy and laborious set-up operations, which means that, normally, each machine is used to wrap a single type of product, thus operating according to a rigid automation system.
In practice, a user who does not possess a different machine for each different product but nevertheless wishes to wrap different products, after establishing the geometrical compatibility between machine and product, continues to use the same work flow even for products different from the product for which the machine""s work flow was originally designed, without making any adaptations to improve the work flow in relation to the product being wrapped.
This leads to two orders of problems: the first is linked to economy of production in terms of unit product cost and machine productivity; the second is linked to the construction and operating costs of the machine itself. Indeed, in the former case, the time scale of the production cycle remains unchanged even when wrapping smaller products, which is obviously anti-economic. Furthermore, since the operating steps follow each other in sequence and at different angular (and time) intervals, the work flow inevitably involves stopping some of the effectors and thus braking the related drive motors. This repeated stopping and starting eventually leads to excessive wear of some of the motors, caused especially by overheating, creating limits on production due essentially to the fact that a few, or even just one, of the drive motors reach critical conditions before the others.
The present invention has for an object to overcome the above mentioned disadvantages by providing a control apparatus capable of intelligently controlling the different operating conditions of different motors, which are driven at instantaneously variable speeds subject only to the requirement of satisfying certain conditions essential to the correct performance of the process cycle.
In accordance with the invention, the above object is achieved by an apparatus for automatically controlling the work flow of a machine as described in the preamble to claim 1, the apparatus comprising: electronic storage means having resident functions that relate the angle of rotation described by a controlling motor with the desired instantaneous position of a controlled effector; means for detecting the actual instantaneous position of the controlled effector; comparator means for determining the positional error of the controlled effector relative to a given expected position upon reaching a certain angle of rotation of the controlling motor; and means for controlling the motor of the controlled effector, which receive the error signal from the comparator means and apply to the motor a corresponding corrective action designed to control and synchronize the angular position of the controlled motor with that of the controlling motor.
If the resident functions are embodied as a discrete set of control conditions, the apparatus can be used to implement a sort of discontinuous, point-to-point control system of the machine""s work flow.
Further, if these control conditions are defined as functions of absolute elementsxe2x80x94outside the machinexe2x80x94and relating, for example, to the characteristic dimensions of the product or of the corresponding package, the control apparatus, if appropriately interfaced with the wrapping machine, permits flexible control of work flows optimized for each type of product and, hence, flexible control of a traditional machine.